CVE-2021-47535

In the Linux kernel, the following vulnerability has been resolved:

drm/msm/a6xx: Allocate enough space for GMU registers

In commit 142639a52a01 (drm/msm/a6xx: fix crashstate capture for A650) we changed a6xx_get_gmu_registers() to read 3 sets of registers. Unfortunately, we didnt change the memory allocation for the array. That leads to a KASAN warning (this was on the chromeos-5.4 kernel, which has the problematic commit backported to it):

BUG: KASAN: slab-out-of-bounds in _a6xx_get_gmu_registers+0x144/0x430 Write of size 8 at addr ffffff80c89432b0 by task A618-worker/209 CPU: 5 PID: 209 Comm: A618-worker Tainted: G W 5.4.156-lockdep #22 Hardware name: Google Lazor Limozeen without Touchscreen (rev5 - rev8) (DT) Call trace: dump_backtrace+0x0/0x248 show_stack+0x20/0x2c dump_stack+0x128/0x1ec print_address_description+0x88/0x4a0 __kasan_report+0xfc/0x120 kasan_report+0x10/0x18 __asan_report_store8_noabort+0x1c/0x24 _a6xx_get_gmu_registers+0x144/0x430 a6xx_gpu_state_get+0x330/0x25d4 msm_gpu_crashstate_capture+0xa0/0x84c recover_worker+0x328/0x838 kthread_worker_fn+0x32c/0x574 kthread+0x2dc/0x39c ret_from_fork+0x10/0x18

Allocated by task 209: __kasan_kmalloc+0xfc/0x1c4 kasan_kmalloc+0xc/0x14 kmem_cache_alloc_trace+0x1f0/0x2a0 a6xx_gpu_state_get+0x164/0x25d4 msm_gpu_crashstate_capture+0xa0/0x84c recover_worker+0x328/0x838 kthread_worker_fn+0x32c/0x574 kthread+0x2dc/0x39c ret_from_fork+0x10/0x18

Weakness

The product writes data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer.

• Be wary that a language’s interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

• Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking.

• D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

• Consider adhering to the following rules when allocating and managing an application’s memory:

• Run or compile the software using features or extensions that randomly arrange the positions of a program’s executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.

• Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as “rebasing” (for Windows) and “prelinking” (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.

• For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].

• Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.

• For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

References

• https://git.kernel.org/stable/c/83e54fcf0b14ca2d869dd37abe1bb6542805f538
• https://git.kernel.org/stable/c/b4d25abf9720b69a03465b09d0d62d1998ed6708
• https://git.kernel.org/stable/c/d646856a600e8635ba498f20b194219b158626e8
• https://git.kernel.org/stable/c/83e54fcf0b14ca2d869dd37abe1bb6542805f538
• https://git.kernel.org/stable/c/b4d25abf9720b69a03465b09d0d62d1998ed6708
• https://git.kernel.org/stable/c/d646856a600e8635ba498f20b194219b158626e8